In order to determine the number of distinct visits, Google Analytics uses a cookie — a small text file stored by the visitor’s browser, which is sent to the site with every request. If the visitor’s browser blocks the cookie, their visits cannot be tracked. Reasons for blocking the cookie include:
The editing system counts all page views. It can determine these with complete accuracy as it is responsible for generating the pages being viewed.
From the page views it determines the number of distinct visits using the referrer header — a piece of information sent by a browser when it requests a page. If the visitor followed a link, the referrer header contains the address of the page containing the link. In other situations (for example, if the visitor has clicked on one of their bookmarks) the referrer header is blank.
If the referrer header is blank or contains the address of a page on another site, the editing system counts the page view as the start of a new visit. If the referrer header contains the address of a page on your site, the editing system counts the page view as the continuation of an existing visit.